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We present a sample of Cepheid variable stars towards M31 based on the firstyear of regular M31 observations of the PS1 survey in the r_P1 and i_P1filters. We describe the selection procedure for Cepheid variable stars fromthe overall variable source sample and develop an automatic classificationscheme using Fourier decomposition and the location of the instability strip.We find 1440 fundamental mode (classical \delta) Cep stars, 126 Cepheids in thefirst overtone mode, and 147 belonging to the Population II types. 296 Cepheidscould not be assigned to one of these classes and 354 Cepheids were found inother surveys. These 2009 Cepheids constitute the largest Cepheid sample in M31known so far and the full catalog is presented in this paper. We brieflydescribe the properties of our sample in its spatial distribution throughoutthe M31 galaxy, in its age properties, and we derive an apparentperiod-luminosity relation (PLR) in our two bands. The Population I Cepheidsnicely follow the dust pattern of the M31 disk, whereas the 147 Type IICepheids are distributed throughout the halo of M31. We outline the timeevolution of the star formation in the major ring found previously and find anage gradient. A comparison of our PLR to previous results indicates a curvatureterm in the PLR.

Properties of M31. II: A Cepheid disk sample derived from the first year of PS1 PAndromeda data